Apparatus for providing short bunches of charged molecular,atomic,or nuclear particles



3,519,942 GED July 7, 1970 R. c. MOBLEY APPARATUS FOR PROVIDING SHORTBUNCHES OF CHAR MOLECULAR, ATOMIC, OR NUCLEAR PARTICLES Filed April 13.1966 3 Sheets-Sheet l OW) mmJDl NN WN mumnom zo @WWW/.Mm

July 7, 1970 R. c. MOBLEY 3,519,942

APPARATUS FOR PROVIDING SHORT BUNCHES OF CHARGED MOLECULAR, ATOMIC, ORNUCLEAR PARTICLES Filed April 15. 196e sheets-sheet 2 l K35 Q \\7//////,

I I l i 1| l SWW/ 5H 52,53 iB-54 35 C- s -y---ETQ i u' ON l l l I I PATHl l :d n l a i 5o l 5o ///M/ INVENTOR. 4 RALPH C. MOBLEY BY GRAY, MASE8.DUNSON ATTORN EYS BWQLWI. www

July 7, i970 R c. MOBLEY 3,519,942

APPARATUS FOR PROVIDING SHORT BUNCHES OF CHARGED MOLECULAR, ATOMIC, ORNUCLEAR PARTICLES Filed April l5, 1966 3 Sheets-Sheet 5 1 5 DRIFT TUBE[L \\i ////////////////4 r/ Gl G2 7 |ON PATH LMTS I ENS l- ENs \v LENS lPOTENTIAL ON GRID 82) R F SIGNAL SOURCE INVENTOR.

RALPH c. MQBLEY (ALL GROUNOS ARE LABORATORY GROUND) BY GRAY, MASE 8.DUNSON ATTORN EYS United States Patent O 3,519,942 APPARATUS FORPROVIDING SHORT BUNCHES F CHARGED MOLECULAR, ATOMIC, OR NU- CLEARPARTICLES Ralph C. Mobley, 2585 Stoodleigh Drive, Rochester, Mich. 48063Filed Apr. 13, 1966, Ser. No. 542,361 Int. Cl. H013' 23/00, 23/34 U.S.Cl. 328-233 26 Claims ABSTRACT OF THE DISCLOSURE lons from a source 8(FIG. 1) are focused `by a lens 9, fed at low velocity in a column 10into the region 1, and al1 are accelerated momentarily to high velocitytherein. Decelerating the ions as they enter the region 2 greatlyforeshortens the column. Similar momentary acceleration of the entirecolumn while in the region 3 and deceleration of the ions upon enteringthe region 4 foreshorten the column 10 still more. Further decelerationin the regions 5 and 6 provides even more foreshortening. The column ismomentarily accelerated in the region 6 just before it enters the region7, where the same rate of acceleration is maintained to add an effectivetime bunching of the column on the target 32.

This invention relates to charged particle bunchers. It has to doparticularly with apparatus for providing short bunches of chargedmolecular, atomic, or nuclear particles such as ions, electrons,positrons, and the like.

Apparatus according to the present invention is especially useful forproviding a very high bunching ratio with ions, while utilizing asubstantially higher proportion of the ions supplied to the apparatusthan has been possible with prior devices. The apparatus can also -bemade much smaller and lighter than the presently known types of ionbunchers.

Typical apparatus according to this invention for providing shortbunches of charged particles may comprise:

(a) Means for providingsaid particles at low velocity to a rst region soas to accumulate and form a column therein of substantial length;

(b) Means for momentarily accelerating all said partices in said columnsubstantially equally to higher velocity in said rst region; and

(c) Means for decelerating each said particle to lower velocity as itenters a second region, and thus to foreshorten said column therein to asmall fraction of its initial length.

Such apparatus may comprise also:

(d) Means for momentarily accelerating all said particles in theforeshortened column substantially equally to higher velocity in a givenregion; and

(e) Means for decelerating each said particle to lower velocity as itenters a subsequent region, and thus to further foreshorten said columnto a small fraction of the length it had just before entering saidsubsequent region.

Apparatus of this invention may comprise also:

(f Means for accelerating all said particles in the furtherforeshortened column substantially equally to higher velocity in a givenregion, immediately before they enter a subsequent region; and

(g) Means for continuing to accelerate all said particles in saidsubsequent region at the same rate of acceleration as that provided bythe means (f), and thus to provide an effective time bunching of saidcolumn on a target beyond said subsequent region substantiallyproportional to the increase in velocity of said column between saidgiven region and said target.

ice

The apparatus may also include other preferred and alternative featuresas disclosed and claimed herein.

In the drawings:

FIG. 1 is a largely schematic and partially sectional view of typicalapparatus and waveforms therein according to the present invention.

FIG. 2 is a largely schematic sectional view of typical electrodes andassociated structure according to this invention.

FIG. 3 is a largely schematic sectional view of other typical electrodesand associated structure and components according to the invention.

FIG. 4 is a largely schematic and partially sectional view of othertypical apparatus according to the invention.

FIG. 5 is a largely schematic and partially sectional view of othertypical apparatus and waveforms therein according to the invention.

FIG. 6 is a view similar to FIG. 5 of still other such typical apparatusand waveforms therein.

FIG. 7 is a schematic diagram of a type of modification that can be madein the circuitry in FIG. l.

FIG. 8 is a schematic diagram of another type of modication that can bemade in the circuitry of FIG. 1.

FIG. l shows a typical embodiment of the invention as designed forbunching positive deuterium ions. Some typical dimensions and electricalquantities are included for convenience to the reader, but the gure islargely schematic and is not drawn to scale. Positive deuterium ions areprovided at high velocity with an energy of about 5 kilo electron volts(kev.) from an ion source 8, such as a Moak type radio frequency ionsource (described in Nucleonics 9 no. 9, 18 (1951) by Moak, Reese andGood., or see Fast Neutron Physics, Part I., by Marion andFowler-Interscience Publishers 1960pages 535-538). The ions are focusedby a lens 9 into a large diameter parallel beam 10, and are deceleratedto a lower velocity with an energy of about electron volts (ev.) beforey entering an equipotential region 1 through a grid 11. This wouldnormally be accomplished by operating the ion source 8 so that theelectrode (probe canal in an R.F. ion source) out of which the positiveions emerge from the source is at a suflciently negative potential thatwhen the ions reach the ground potential of the grid 11 they have only100 ev. left. For example, -1900 volts for positive emerging ions with akinetic energy of 2000 ev. (For negative ions the polarity of theretarding potential would be reversed.) The ions thus enter the region 1through grid 11 at a relatively low energy and hence low velocity.

For about 7/s microsecond ions are allowed to accumulate, partiallyfilling the region 1 and forming a column 10 of monoenergetic ions about8.6 centimeters long. A 1A; microsecond 13 kilovolt positive pulse isthen applied from a pulse generator 21 through a capacitor 22 to thegrid 11, creating a substantially uniform electric field between thegrids 11 and 12 and causing all ions in the region 1 to acceleratesubstantially equally. Just before the front of the column 10 reachesthe grid 12 the pulse on the grid 11 terminates. The region 1 returns toits normal equipotential condition, with a resistor 23 and a diode 23'maintaining the grids 11 and 12 at substantially the same potential inthe intervals between pulses. All ions now move at the same constanthigher velocity with an energy of about 2500 electron volts, and thelength of the column or bunch 10 remains constant at about 8.6 cm.

The ions exit from the region 1 through the grid 12 and enter a driftand deceleration region 2. In the region 2 the i-ons are deceleratedindividually to about 100 ev. by the 2400 volt potential differencebetween the grids 12 and 13 which is provided by a direct voltage supply24 the negative terminal of which is connected to the terminal ground25, to which the grid 12 also is connected. The

positive terminal of the power supply 24 is connected to a grid 14 andthrough a resistor 26 and a diode 26 to the grid 13. The reduction inkinetic energy by 25 reduces the ion velocity by and causes aforeshortening of the bunch as it accumulates in the region 3 by afactor of 5. The length of the 100 ev. ion column or bunch 10 in theregion 3 is then about 1.7 cm. The 11 cm. spacing between the grids 12and 13 is also chosen to delay the entrance of the bunch 10 into theregion 3 until just before the arrival of the next pulse provided fromthe pulse generator 21 through a capacitor 27 to the grid 13. The region2 between the grids 12 and 13 then serves both to decelerate the ionsand to delay the bunch 10 for proper phasing upon entering the region 3.

The uniform acceleration -of all the ions in the region 3 to about 6300ev. is followed by separate or individual deceleration in the region 4to an energy of about 100 ev. causing an additional bunching of about7.9. The proper retarding potential is furnished by a direct voltagesupply 28 through a resistance 29 to the grid 15. A resistancer30 isconnected at one end to the resistance 29 and the voltage supply 28 andat the opposite end to the grid 16. A diode 30 is connected in parallelwith the resistor 30. Thus the grids and 16 are maintained atsubstantially the same potential, in the intervals between pulses, toprovide an equipotential drift region 5 to achieve proper phasing of thebunch 10 as it enters the region 6.

A third stage of acceleration is provided in the region 6 by anotherpositive voltage pulse provided by the generator 21 through a capacitor31 to the grid 16. The twice foreshortened column 10 enters theaccelerator tube 7 at the peak of the pulse on the grid 16, at whichtime the gradient in the region 6 matches that of the accelerator tube7, making possible an additional effective time bunching 4of about 55for an accelerator potential of 300,000 volts (70 for 500,000 volts).This effective further biinching comes about because the gradient in theregion 6 is chosen to match that of the accelerator tube 7 as the bunch10 leaves the region 6. The bunch length of 0.2 2 cm., therefore, doesnot change as the bunch proceeds down the accelerator tube 7. Arrivaltime on the target 32, however, decreases in proportion to the ratio offinal to initial velocity. For an initial ion energy of 100 ev'.r in theregion 6 and a final ion energy of 300,000 ev. .upon exiting from theaccelerator tube 7, the velocity increases by a factor of 300,000/100=54.7 with an effective time bunching to this same extent. Theoverall bunching then is 5 7.9 54.7=2l60. For an initial bunch of 7Amicrosecond, the nal burst length on the target 32 is about 7A:microsecond/2l60=0.4l nanosecond. With a pulse :repeltition rate of lmc./sec., ideally 87 percent of ion source output would be bunched. y

This embodiment of the invention, therefore, comprises a light compactbuncher of very high bun'ching ratio and of very high ion sourceutilization eiciency, suitable for installation in a wide variety ofaccelerators and tandem injectors. Variation of the initial ion energyand pulse height make a given buncher also adaptable to a wide range ofion masses.

In the above explanation the effects of a number of factors have beenomitted for simplicity. They are: energy spread in the ions from the ionsource, space charge effects, energy spread introduced by the thirdbunching stage, and ion losses to the various grids.

The energy spread of the ion source 8 tends to be magnied in each stageby the bunching ratio of that stage. Because of the small extent (2.2mm.) of the foreshortened bunch 10 by the time it reaches the region 5,ions of slightly different energies in the region 1 will have becomeseparated physically in the region 5 with the higher energy ions beingcloser to the grid 16 and the lower energy ions closer to the grid 15.Application of a small square wave positive pulse to the grid 15 from apulse generator 33 through a capacitor 34 after all the ions are in theregion 5, and maintenance of the pulse until all the ions have exitedthrough the grid 16 into the region 6, gives more energy to the lowenergy ions than to the high energy ions. It is thus possible tocompensate, at least to first order, for the ion source energy spreadand at the same time, to some extent, for the effects of space chargespreading. The use of a large diameter parallel ion beam throughout thebuncher also minimizes space charge effects. Because the ions aresubjected to a large electrical gradient after they have entered theregion 6 and because, unlike the preceding stages, this gradient ismaintained until the ions exit from the accelerator tube 7, the ions atthe rear of the bunch 10 fall through a larger potential difference thanthe ions in the front of the bunch by an amount equal to the gradient inthe region 6 times the length of the bunch 1G therein. In the aboveexample this potential difference is 0.22 cm. 4.65 kv./crn.=1.02 kv.

Because the energy spread is linearly distributed along the bunch it canbe removed if necessary as the ions exit from the accelerator tube by asmall amount of inverse velocity modulation at that point. This can lbeaccomplished in at least two different ways, either lby the inverse ofklystron velocity modulation or by the inverse of the bunching action ofregion 6 of FIG. 1.

The first technique is illustrated in FIG. 5 for the parameters of theprevious example, i.e., 300 kev. deuterons which would have a velocityof 5.4 108 cm./sec. The ion bunch 10 emerging from the accelerator tube7 is focused by a lens 71 (such as a stron-g focusing electrostic ormagnetic quadrupole pair) to a focus at the center of the gap G1. A lens72 refocuses the bunch at the center of the gap G2 and finally a lens 73focuses the bunch 10 on the target 32, which is not shown in FIG. 5. Thebunch 10 also is not shown in FIG. 5, only the envelope of its path asdashed lines. The drift tube 74 is driven by a radio frequency signalsource which for example could have as one possible combination anamplitude of 0.51 kv. at a frequency of 405 mc./sec. This voltageappearing across the gaps G1 and G2, if properly phased with the passinglbunch, will remove the 1.02 kv. energy spread introduced as the bunch10 emerges from the accelerator tube 7.

This comes about in the following manner. If the gaps G1 and G2 aresmall (say 1 mm., which is a practicable separation for smoothlypolished and rounded surfaces in a good vacuum at the voltage andfrequency mentioned), an ion traversing a gap will acquire or lose anamount of energy equal to the average potential across the gap duringits passage across the gap. If the gap transit time is limited to the 27of the R.F. cycle in this example or less, then if the ion bunch passageis timed to traverse the gap during the essentially linear portion ofthe R.F. cycle such as from a to Iz for G1 and from c to for G2 (FIG.5), then the average potential for any given ion as it traverses the gapis essentially that which exists across the gap as the ion crosses thecenter plane of the gap. For 300 kev. deuteron ions which travel atapproximately 5.4 108 cm./ sec., the time t required to traverse thecenter plane of the gap would be t=extent of bunch/v=0.22 cm./5.4:m./sec.=4.1 10-10 sec. If the energy spread of the ion bunch is 1.02kev., as in the example, then if each gap removes half this spread, thevoltage across each gap will have to change by 510 Volts as the ions inthe bunch cross the center plane of the gap- Considering the rst gap G1,and remembering that thtx ions at the front of the bunch 10 are lower inenergy than those at the Iback by 1.02 kev., then the leading ions oughtto see -255 volts on the drift tube 74 (point a on the R.F. cycle inFIG. 5) thus gaining approximately this energy, while those at the rearof the bunch 10 ought to see +255 volts on the drift tube 74 (point b),thus losing this amount of energy. The difference between points a and bthen being 510 volts. If this swing occurs between 0=30 and +30 of thesine Wave on the drift tube 74 as the bunch 10 passes through the gapG1, then this corresponds to 1A; of the R.F. cycle and since this mustalso be the bunch transit time through the center plane of the gap G1 or4.1 1010 sec., then the R.F. period T =6 4.1 10r1 sec.=2.46 109 sec. andthe frequency =T1=405 megacycles. Also, since sin 0:1/2, the amplitudeof the R.F. voltage would be 2X255=5l0 volts. Other combinatons are alsopossible, for instance, the transit time 4.1 10-10 sec. might be madeequal to 1/12 cycle, whence the frequency would be 202.5 mc. and theamplitude approximately twice or 1.02 kv. or 101.25 mc. and 2.04 kv.,etc.

Choosing the initial combination, 405 mc., and 510 volts for furthercalculation, it is now necessary to remove the other 510 ev. energyspread from the bunch as it emerges through the gap G2. For this tooccur it is necessary for the drift tube voltage to swing from c to d orfrom +255 volts to -255 volts as the bunch 10 crosses the center planeof the gap G2. This shift from the front side to the back side oftheR.F. cycle in going from the gap G1 to the gap G2 is necessary becausethe direction of the electric field is opposite in the two gaps for agiven voltage polarity on the drift tube 74. To assure that this be thecase, the ion bunch 10 must spend (n+1/2) cycles in traversing thedistance L between the centers of the two gaps, where n=0, 1, 2, etc.,i.e., a whole number. Now in (n+1/2) cycles, the ions will travel adistance L=(n-I1/2) TX v=(n'l1/2) 2.4 109 sec. 5.4 l08 cm./sec.=(n+1/2)1.32 crn. If we let n=20, then L=27 cm., a quite reasonable value.

The reason for the three lenses in FIG. 5 is to permit the gaps G1 andG2 and their aperture to be as small as possible. Also, since these gapswill generate quite strong time varying electrostatic lenses, a beamcrossover in the center of such lenses will nullify this effect. Thiscan be readily seen from an optical analogy.

An alternative possibility is to form the gaps by two very closelyspaced (l mm.) grids. The lenses would not then be necessary. Whetherlarge grids with such small spacing (1 mm.) between grids and the muchcloser spacing of wires in a given grid (perhaps 0.1 mm. or 0.1 of thegap) needed to make the grids sufficiently smooth equipotential planesfor this gap spacing are practical either physically or electrically isnot clear. The arrangement in FIG. 5 appears to be more practical.

With this particular arrangement the length L is related to the velocityv of the ions and hence to their energy and to the number of R.F. cyclesn the ions spend in the drift tube. For a fixed buncher frequency (themost convenient operating condition technically at present) and a fixeddrift tube length L, the velocity of the ions and hence their energycould be changed only in discrete steps corresponding to changes in thevalue of n by l. In the previous example, this Would be a 4 percentchange in velocity or 8 percent change in energy. Alternatively keepingn constant and changing the frequency from 405 mc. (405th harmonic ofthe buncher frequency) to 406 mc. (406th harmonic) would correspond to a0.25 percent change in velocity and hence a 0.5 percent change inpossible ion energy. Varying both n and f gives many more possibilities,but all corresponding to discrete ion energies. Modifcation of thebuncher oscillator to allow a small change in frequency (1 percent wouldbe more than enough) or construction of the drift tube so that L couldbe varied by l0 percent would make it possible to handle ions of anyenergy throughout the design range of this energy demodulator.

The sine wave technique for prevention of velocity modulation of the ionbunch upon emerging from the accelerator is essentially that of region 6of the buncher (FIG. 1) in reverse with a sine wave substituting for thepulse. FIG. 6 shows a way in which this technique can be used toprevenlt the 1.02 kev. energy modulation in the ion bunch. A firstgridded electrode 81 is grounded to the laboratory ground while a secondgridded electrode 82 is driven with a sine wave from the R.F. signalgenerator 83. A diode 84 serves to bias the grid 82 so that itspotential only swings down from zero as in the adjacent graph in FIG. 6.The signal generator 83 is so phased with respect to the arrival time ofthe ion bunches 10, that the bunches 10 enter region y80 (between grids81 and 82) when the voltage on the grid 82 is at e, the bottom of itsswing, and exit from the region through the grid 82 when the voltage onit is zero (point f). Now if the gradient in the region 80 matches thatof the accelerator tube 7 when the bunch 10 enters and drops to zerojust as it leaves, then all ions in the bunch 10 are subject to the sameelectrostatic forces at all times and will therefore all have the samevelocity upon emerging into the region 90.

This technique does not remove velocity modulation, it doesnt allow itto start in the first place. As a specific example, let the frequency ofthe R.F. signal source be 10() megacycles/sec. Clearly it is necessarythat this frequency be some integral multiple of the buncher frequencyof l megacycle/ sec. if proper phasing is to be maintained between thesignal source and bunch entrance into the region 80. For f= mc./ sec.the bunch will move 5.4 10S cm./sec.

#103 cycles/see. :2'7 cm' itx during one-half cycle of the voltage onthe grid 82. The separation s between the grids 81 and 82 then needs tobe 2.7 cm. In order to match the accelerator tube gradient of 4.65 kv./cm., the peak to peak R.F. voltage would have to be 2.7 cm. 4.65kv./cm.=l2.5 kv. A peak R.F. voltage of 6.25 kv. would therefore beneeded. In arriving at this result it is assumed that the electric fieldbetween the grids 81 and 82 is uniform.

The entire buncher and associated system from the ion source y8 throughthe accelerator tube 7 in FIG. 1 are all operated under high vacuum(10-5 mm. of mercury or better) maintained by high vacuum pumps(diffusion, getter, etc., not shown). The insulating spacers 35 betweenthe electrodes 11-417 normally are used both for mechanical support andas part of the wall `36 for maintaining the vacuum in the system. Theaccelerator tube 7 is similarly constructed with insulating sections 37between the electrodes. Resistances 38 are connected from each electrode39 to the next, to divide the potential evenly from the high voltageterminal at 25 to the laboratory ground 40 of the accelerator tube 7.

A preferred form of the pulse generator 21 is disclosed and claimed inthe copending United States patent application of the present inventor,Ser. No. 663,820, filed Aug. 28, 1967. Part of the circuit is shownschematically in FIG. l. The circuit is basically a push-pull amplifierwith a high Q delay line load. The amplifier tubes are drivenalternately to cut off by short negative pulses, the drive to the twotubes being periodic at a repetition rate of l mc./ sec., chosen tomatch the transit time down the delay line and back. A pulse travelingalong the line is in a sense resonantly reinforced by first one tube andthen the other as it reaches each end of the line and is reflected.Since the output voltage pulse is substantially sinusoidal in waveformand is delivered from the generator 21 during only one-eighth of eachcycle, the B+ voltage from the power supply 41 theoretically could be aslow as one-sixteenth of the voltage at the peak of the output pulse.Because of losses, the supply voltage must be higher than that, butstill only a small fraction of the pulse voltage.

The waveform and phase of the pulses at the grid 13 are shown in theupper curve 42 at the bottom of FIG. l, while the waveform and phase ofthe pulses at the grids 11 and 16 are shown on the same time axis in thelower curve 43. Of course the output over a complete cycle is notsinusoidal. This condition for this particular pulse generator is causedby the cutoff characteristics of the delay line. The shape of the pulseis not important as long as it is short enough and has the properaverage value. An 8 7 kv. square pulse would do equally well. Since thecondenser 22 blocks D-C, and since the resistor 23 is tied from the grid11 to the terminal ground 25, the average Voltage on the grid 11 iszero. Since the pulse generator 21 forces the grid 11 to rise 13 kv.during 1/s microsec. in the example (average value of the pulse is 8 kv.and is thus equivalent to a square pulse of this height), then for thepotential to average zero it would actually swing up to +l2,000 volts,then down to -1,000 volts forthe remaining %p. sec. for a total swing of13 kv. For best operation the grid y11 should swing down to, but notbelow the grid 12 or zero. Similarly for the grids 13 and 14 and thegrids 16 and 17, respectively. This can be accomplished in either of twoways.

A high voltage diode, preferably a vacuum diode, can be placed acrosseach resistor 23, 26, and as shown in FIG. 1. This .is probably thesimplest solution. It can be arranged to hold the grid 11 within a fewvolts of terminal ground. By connecting the anode of the diode 23 to aslightly positive adjustable voltage with respect to terminal ground, asat 23" in FIG. 7, the grid 11 can be made to assume an average potentialvery close to zero between pulses. Similarly for the grids 13, 14, and15, 16. With capacitance 22 of 1000 paf. or more, the voltage variationbetween pulses is 1.5 volts or less, which is an acceptable value.

An alternative is to insert a power supply of +1000 volts in series witheach resistor 23, 26, and 30, as shown at 23 in FIG. 8 for the resistor23. Here the grid 11 has an average potential of +1000 volts swinging upto +13 kv. and down to zero. Again with a capacitance 22 of 1000 gaf. ormore, the time constant of the circuit is 1 millisecond or more and thevoltage change on the grid 11 between pulses varies only from 0.7 to+0.7 volt as a maximum, again a very acceptable value.

The invention is applicable generally to charged particles of any type,such as electrons, positions, etc., as well as ions. The configurationshown in FIG. 1 illustrates the bunching principle involved usingdeuteron ions as the charged particles. This is a form of ion buncher ofgreat practical value to nuclear physicists. Changing the pulse heightand initial particle energy on the entering grid 11 makes it possible tobunch (at separate times) particles of widely different masses. Goingfrom deuteron ions to electrons or positions requires such a largechange in the parameters, however, that a change in dimensions of thebuncher is advisable.

The grids 11-17 preferably comprise parallel wires carefully alignedoptically from grid to grid along the ion paths in order to make all thewires in each grid following the grid 11 fall in the shadow of thecorresponding wires in the preceding grid. Grids are advantageous toassure the uniform fields between electrodes needed for the bunchingprocess.

A truly uniform field can be had between plane electrodes only if theyare infinite in extent. Such electrodes preferably should extendsufficiently far from the ion path radially to assure an essentiallyuniform field. This requires electrode diameters of several times theseparation between electrodes, which would be rather unwieldly.

It is preferable to thicken the electrode as the radial distance fromthe beam path increases, as shown schematically in FIG. 2. This placesthe opposed inner surfaces 44 and 45 of the electrodes 11' and 12somewhat closer together near their edges to compensate for the tendencyof the field to fall off with distance from the center of fiatelectrodes of finite extent. The insulator also serves as part of thevacuum wall needed for the buncher, which operates under the relativelyhigh vacuum characteristic of accelerators. Reducing the electrode sizein this way has, beyond the obvious one of compactness, which is veryimportant, the additional advantage of reducing nterelectrode capacityand the loading it might throw on the pulse generator that drives it.

Another way to attain a uniform field is to use flat electrodes with apoor conductor in place of the insulator. An equivalent alternative wayis to sub-divide the region 3 with additional ring electrodes 46 andwith resistors 47 Vbetween them as shown in FIG. 3. Both of these latterarrangements suffer from the large amounts of power that would bedissipated in the resistors 47 or the poor conductor. One might usecondensers rather than resistors, but there would then be large amountsof reactive current. Either would heavily load the pulse generator. Thearrangement in FIG. 2 is therefore generally preferred, but otherconfiguration may be useful for some purposes.

Besides helping to form a uniform field in the region 1 (FIG. l) duringthe first phase of bunching, the grid of the electrode 11 also servesthe very important function of preventing ions from the source 8 fromentering the region 1 during the pulse (by repelling them back towardthe source). This chopping action, if not performed by the grid 11,would have to be (and could be) performed by some other means to theleft of the region 1. If additional ions were to enter the region 1while the pulse is on the grid 11, the additional ions would acquirevariable amounts of energy and therefore would not bunch properly.Ideally Ms of the ions are bunched and 1/8 thrown away in the examplegiven. Bunching Vs of the ions from the source is a superior achievementnot approached by any other known bunching apparatus.

The presence of the grids in the electrodes 11-17 in FIG. 1 causes someloss of ions traversing the buncher. Except possibly for the grid in theelectrode 11 Iwith its chopping action, all the other grids in thebuncher could probably be eliminated by increasing the spacing betweenthe electrodes considerably, and applying pulses only when the bunch ofions 10 was far enough from the electrodes both at the beginning and theend of the pulse to be in a region of essentially uniform field duringthe' pulse. Both the acceleration regions such as 1 and the decelerationregions such as 2 could probably be made minimal in length through thearrangement of FIG. 3. In any case a buncher with such electrodes wouldtend to be longer than that of FIG. 1 and the transition fromacceleration to deceleration regions would be accompanied by strongelectrostatic lens action on the ion bunch 10 because of the curvaturein the equipotential surfaces produced by the change in potentialbetween regions not having grids between them. The lens effect might beadequately offset by an axial (solenoidal) magnetic field directed alongthe beam axis of the buncher or by the stronger focusing action providedby ring magnets arranged coaxially with the beam poth with theirmagnetic field directions alternating axially from magnet to magnet (asin the permanent magnet focusing utilized in traveling wave tubes).

Another, but probably less satisfactory, solution would be to usequadrupole focusing magnets distributed along the beam path as with thering magnets mentioned above. Quadrupole focusing magnets are widelyused with accelerators in nuclear physics to focus beams.

When a completely gridless buncher can be successfully built is notclear. If all grids cannot be. eliminated, however, some very likely canbe. It then becomes a question of whether the added complexity of thecompensation techniques outweighs the added efiiciency obtained withoutgrids.

True physical bunching in the apparatus of FIG. 1 occurs only in theregions 1 through 4. In the region 6 and the accelerator tube 7 theprocess is quite different. Here the bunch 10 does not get shorterphysically as it moves from the region 6 into and down the acceleratortube 7, but remains the same physical length and merely moves faster.The effect in terms of arrival time of the ion bunch 10 on the target 32at the output of the accelerator 7, however, is the same. The faster itgoes, the shorter the arrival time. This is not Klystron bunching.Klystron bunching relies on velocity modulation, which does not existhere at all until the bunch 10 emerges from the accelerator tube 7 andthen what does come about isl not appreciable.

Where a bunch of particles is sufficiently short, as in the region 5,further bunching can be accomplished using ordinary sine waves insteadof pulses. A means for doing that is shown in FIG. 4. Electrodes 51 and53 are lgrounded. The electrode 52 is driven by a sine wave oscillator60 through a coupling capacitor 61, and is biased by a diode 62, so thatit swings only positive from ground. The particle bunch 50, being short,is admitted during the time the electrode 52 is at ground potential.With the frequency and voltage of the sine wave and the spacing of theelectrodes 52 and 53 properly chosen, the bunch 50 is accelerated fromthe electrode 52` toward the electrode 53 and arrives at the electrode53 just as the electrode 52 swings back to ground potential. The bunch50 then exits from the region A when the field in this region is zero.

Deceleration in the fixed potential retarding region B then causesbunching as in the region 2 of FIG. 1. With the spacing of theelectrodes 53 and 54 in the region B properly chosen the bunch exitsfrom the region B through the electrodes 54 and 55 into the region Cwhen the electrode 55 is at its minimum potential, which is equal to thepotential of the electrode 54. At the moment the particle bunch 50enters, the region C has a potential equal to that of the voltage of thebias supply 63 as needed to decelerate the ions in the region B to thevelocity they had on entering the region A. The process is then repeatedwith the region C playing the part of the region A and the followingregion that of the region B, etc. A diode 64 biases the electrode 55 sothat it swings only positive with respect to the bias supply 63 andhence also with respect to the electrodes S4 and 56. Coupling capacitors65 and 66 connect the alternating voltage source 60 to the electrode 55and subsequent electrodes and a capacitor 67 bypasses alternatingcurrent around the bias supply 63. The bunching in the present apparatusis clearly different from that which occurs in a cyclotron or a linearaccelerator. Inthe latter devices all regions or sections of theIaccelerator are at the same average D-C electrical potential (zero)with acceleration and so-called phase `bunching occurring under theaction of R-F fields, only, which at the same time produce progressivelyincreasing velocities. In the present sine-wave buncher the average D-Cpotential is increased from stage to stage by providing a higher biasvoltage on each successive stage (as in FIG. l), while the averagevelocity throughout the buncher remains constant. This form of thebuncher may be especially suitable for electron bunching. Also, a coupleof stages of this type Imight profitably be used following the region 5of FIG. 1, where the ion bunch is very short.

Of course, voltages having polarities opposite to those shown in thedrawings can be applied to the electrodes at the other ends of thevarious regions to provide the same actions in those regions. Forexample, in FIG. 1, negative pulses can be applied to the electrodes 12,14 and 17, instead of positive pulses to the electrodes 11, 13, and 16,to accelerate the column 10 in the regions 1, 3, and 6, respectively.

While the forms of the invention herein disclosed constitute presentlypreferred embodiments, many others are possible. It is not intendedherein to mention all of the possible equivalent forms or ramificationsof the invention. It is to be understood that the terms used herein aremerely descriptive rather than limiting, and that various changes may bemade without departing from the spirit or scope of the invention.

What is claimed is:

1. Apparatus for providing short bunches of charged particlescomprising:

(a) means for providing said particles at low velocity to a first regionso as to accumulate and form a column therein of substantial length;

(b) means for momentarily accelerating all said particles in said columnsubstantially equally to higher velocity in said first region;

(c) means for decelerating each said particle to lower velocity as itenters a second region, and thus to foreshorten said column therein to asmall fraction of its initial length;

(d) means for momentarily accelerating all said particles in theforeshortened column substantially equally to higher velocity in a givenregion;

(e) means for decelerating each said particle to lower velocity as itenters a subsequent region, and thus to further foreshorten said columnto a small fraction of the length it had just before entering saidsubsequent region;

(f) means for accelerating all said particles in the furtherforeshortened column substantially equally to higher velocity in a givenregion immediately before they enter a subsequent region; and

(g) means for continuing to accelerate all said particles in saidsubsequent region at the same rate of acceleration as that provided bythe means (f), and thus to provide an effective time bunching of saidcolumn on a target beyond said subsequent region substantiallyproportional to the increase in velocity of said column between saidgiven region and said target.

2. Apparatus for providing short bunches of charged molecular, atomic,or nuclear particles comprising:

(a) means for providing said particles at LOW VE- LOCITY in a parallelbeam of large diameter through an opening in a first electrode in saidapparatus into a FIRST REGION, lbetween said first electrode and asecond electrode spaced therefrom in the direction of movement of saidbeam;

(b) means for maintaining said first and second electrodes normally atsubstantially EQUAL POTEN- TIAL to permit said particles to accumulateand form within said first region a column of said particles at leastabout one-half as long as said first region;

(c) means for applying to said first electrode a large electrical AIDINGPULSE to create a substantially uniform electric field between saidfirst and second electrodes when said column has been formed and thus toaccelerate all said particles substantially equally, said pulse endingjust before the forward end of said column reaches said secondelectrode, so that substantially all said particles move through anopening in said. second electrode at substantially equal and constanthigher velocity, and with the length of said column remainingsubstantially constant, into a SECOND REGION, between said secondelectrode and a third electrode spaced therefrom in the direction ofmovement of said particles; and

(d) means for providing a RETARDING POTEN- TIAL DIFFERENCE between saidthird and second electrodes to decelerate said particles to lowervelocity, and thus to foreshorten said column to a small fraction of itsinitial length, as said column moves through said second region andpasses through an opening in said third electrode into a THIRD REGION,between said third electrode and a fourth electrode spaced therefrom inthe direction of movement of said particles.

3. Apparatus as in claim 2, comprising also:

(e) means for maintaining said third and fourth electrodes atsubstantially EQUAL POTENTIAL as said column enters said third region;

(f) means for applying to said third electrode a large electrical AIDINGPULSE to create a substantially uniform electric field between saidthird and fourth electrodes when said column has passed through saidthird electrode and thus to accelerate all said particles substantiallyequally, said pulse ending just before the forward end of said column ofparticles reaches said fourth electrode, so that substantially all saidparticles move through an opening in said fourth electrode atsubstantially equal and constant higher velocity, and with the length ofsaid column remaining substantially constant, into a FOURTH REGION,between said fourth electrode and a fifth electrode spaced therefrom inthe direction of movement of said particles;

(g) means for providing a RETARDING POTEN- TIAL DIFFERENCE between saidfifth and fourth electrodes to decelerate,said particles to lowervelocity, and thus to further foreshorten said column as it movesthrough an opening in said fifth electrode into a FIFTH REGION, betweensaid fifth electrode and a sixth electrode spaced therefrom in thedirection of movement of said particles; and

(h) means for maintaining said fifth and sixth electrodes atsubstantially EQUAL POTENTIAL to continue to foreshorten said column asit enters said iifth region and to maintain it at a constant length asit moves further through said fifth region and through an opening insaid sixth electrode into a SIXTH REGION between said sixth electrodeand a seventh electrode spaced therefrom in the direction of movement ofsaid particles.

4. Apparatus as in claim 3, comprising also:

(i) means for maintaining said sixth and seventh electrodes atsubstantially EQUAL POTENTIAL as said column enters said sixth region;and

(j) means for applying to said sixth electrode a large electrical AIDINGPULSE to create a substantially uniform electric `ield between saidsixth and seventh electrodes when said column has passed through saidsixth electrode and thus to accelerate all said particles substantiallyequally, so that when said pulse reaches its peak said column movesthrough an opening in said seventh electrode into an ACCELERA- TINGDEVICE having a potential gradient in the direction of movement of saidparticles substantially EQUAL to the POTENTIAL GRADIENT present betweensaid sixth and seventh electrodes when said pulse is at its peak, thematching of said potential gradients maintaining the length of saidcolumn substantially constant and providing an effective time bunchingof said column on a target substantially proportional to the increase invelocity of said column of particles between said fifth region and saidtarget. S. Apparatus as in claim 2, wherein a GRID of parallel wires isprovided across the opening in said first electrode.

y6. Apparatus as in claim 4, wherein a similar grid of parallel wires isprovided across the opening in each said electrode, the correspondingWires of all said grids being parallel and in register as viewed in thedirection of movement of said particles.

7. Apparatus as in claim 3, wherein means are provided for applying aSMALL electrical AIDING PULSE to said fifth electrode when said columnhas passed through said fifth electrode and for maintaining said pulseuntil said column has passed through said sixth electrode into saidsixth region; thus to substantially COMPENSATE for any spreadingtheretofore, between particles having different energies, by impartingmore energy to the particles having lower energy than to those havinghigher energy.

8. Apparatus as in claim 7, wherein said pulse has substantiallyconstant amplitude.

9. Apparatus as in claim 4, comprising also means for providing inversevelocity modulation in said column after it leaves said acceleratingdevice, to substantially eliminate any differences in velocity betweenthe particles in said column.

10. Apparatus as in claim 9, wherein said means for providing inversevelocity modulation comprises means for focusing the particles in saidcolumn to a point; and means for providing, in a region having saidfocus point substantially at its center, an electric yfield that variessubstantially linearly from a predetermined accelerating potentialgradient, as said column enters said region, to a predetermineddecelerating potential gradient, as said column leaves said region.

11. Apparatus as in claim 10, comprising also at least one furthercombination of said focusing means and said electric field providingmeans.

12. Apparatus as in clairn 4, comprising also means for substantiallyeliminating any differences in velocity between the particles in saidcolumn as saidY column leaves said accelerating device.

13. Apparatus as in claim 12, wherein said difference eliminating meanscomprises means for providing, in a region adjacent the exit end of saidaccelerating device, an electric field that varies from a potentialgradient that is equal to the potenial gradient in said acceleratingdevice, as said column enters said region, to zero, as said columnleaves said region.

14. Apparatus as in claim 4, wherein said electrodes are conductive andsaid means (b), (e), (h), and (i) comprise RESISTANCES and diodesconnected between said respective electrode for maintaining themnormally at substantially equal potential.

15. Apparatus as in claim 4, wherein said PULSES in (c), (f), and (j)have the same polarity as the charge on said particles.

16. Apparatus as in claim 4, wherein said pulses of (C), (f), and (j)are REPEATED PERIODICALLY and the repetition rate, amplitude, andduration thereof and the velocities of said particles and lengths of allsaid regions in said apparatus are such that successive columns ofparticles are formed and moved periodically through said apparatus withthe proper phasing during their movement as specified in claim 6.

17. Apparatus as in claim 16, wherein said pulses of (c), (f), and (j)have a DURATION of about 0.1 to 0.15 of the period between successivepulses.

18. Apparatus as in claim 4, wherein the amplitude of said pulses of(c), (f), and (j) and the potential differr' encesof (d) and (g) aresuch as to respectively increase and decrease the velocity of theparticles sufficiently to FORESHORTEN said column by a factor of atleast about 5 in said second region and by a factor of at least about 8in said fourth region, and to provide an effective TIME BUNCHING on saidtarget of at least about 50; and thus to provide a total bunching of atleast about 2000.

19. Apparatus as in claim 2, wherein said COLUMN in said FIRST REGION isabout 0.6 to 0.8 as long as said first region.

20. Apparatus as in claim 19, wherein the diameter of said column equalsabout 0.2 to 0.5 of its length.

21. Apparatus as in claim 2, wherein said rst and second ELECTRODES arepositioned transverse to, and coaxial with, said column of particles,and are so shaped that the SPACING between their opposed surfacesdecreases substantially with increasing distance from said column.

22. Apparatus as in claim 4, wherein said electrodes are positionedtransverse to, and coaxial with, said column of particles, and are soshaped that the spacing between their respective pairs of opposedsurfaces in said first, second, third, fifth, and sixth regionsdecreases substantially with increasing distance from said column.

23. Apparatus as in claim 2, including at least one combination offurther foreshortening means, each said combination comprising:

(a) means for accelerating all said particles in the foreshortenedcolumn substantially equally to higher velocity in a given region; and

(b) means for decelerating said particles to lower velocity as theyenter a subsequent region.

24. Apparatus as in claim 23, wherein each said region includes anupstream electrode and a downstream electrode and:

(c) wherein each said accelerating means (a) comprises means forapplying to the upstream electrode of said given region a largeelectrical aiding voltage to create a substantially uniform electricIfield between the upstream and downstream electrodes of said givenregion when said column has passed through said upstream electrode, saidvoltage ending just before the forward end of said column reaches saiddownstream electrode; and

(d) wherein each decelerating means (b) comprises means for providing aretarding potential difference between the downstream and upstreamelectrodes of said subsequent region.

25. Apparatus as in claim 24, wherein said means for applying an aidingvoltage comprises a source of sinusoidal alternating voltage andhalf-wave rectication means connected between said source and saidelectrodes.

26. Apparatus as in claim 25, wherein said source of alternating voltageis connected to a capacitor in series with said electrodes and saidrectification means comprises a unidirectional conducting deviceconnected in parallel with said electrodes.

References Cited UNITED STATES PATENTS 3,333,142 7/1967 Takeda et al.B15-5.41 X

ROBERT SEGAL, Primary Examiner U.S. Cl. X.R. 313-63

